Conventional buck mode (switching mode) converters use metal oxide semiconductor (MOS) transistors, for example, p-channel MOS (PMOS) or n-channel MOS (NMOS) devices, as switching elements. In the case of a switching mode converter that operates at output voltage, the switching mode converter increases the duty cycle D in order to increase the ON-time of a switching element when input voltage drops. In this case, the duty cycle D may be defined as a value that is obtained by multiplying the ON-time of the switching element by the switching frequency of the switching element.
The switching mode converter is limited to a specific maximum duty cycle. In the case of a buck converter that uses a PMOS as a switching element, the duty cycle may be increased by 100% because the gate of the PMOS can be simply pulled down to a ground GND.
However, although a PMOS output device has higher performance than an NMOS device, the PMOS output device is disadvantageous in that the cost thereof is higher than the NMOS device.
When an NMOS is used as a switching element, a method of applying a voltage, which is higher than that applied to the source of the NMOS, to the gate of the NMOS is required. According to conventional technology, a higher voltage is applied to the gate using a bootstrap capacitor and a diode. That is, a conventional switching mode converter illustrated in FIG. 1 includes a high-side drive unit 20, a low-side drive unit 22, a high-side switch transistor 24, a low-side switch transistor 26, a diode 28, a bootstrap capacitor 30, an inductor 32, an output capacitor 34, an output load resistor 36, and an input voltage VIN.
The conventional technology is disadvantageous in that the converter should keep the duty cycle lower than the 100% duty cycle in order to recharge the bootstrap capacitor. Accordingly, when the 100% duty cycle is required because the input supply voltage drops, a problem arises in that some components cannot operate.
In order to overcome the above problem, another conventional technology employs a high-power charge pump in order to provide the complete high-side gate drive regardless of whether a 100% duty cycle is applied. However, the high-power charge pump is disadvantageous in that it is very expensive in the silicon region (or on a silicon basis), has very high cost because additional external capacitors are used, and is subject to many external influences.
Furthermore, since the conventional technology using a high-power charge pump is designed to allow the amount of charge transferred from the charge pump to be large, the gate voltage of the high-side switch may increase to a voltage above a required voltage, with the result that there is a strong possibility of the high-side switching element being damaged.
Accordingly, a method of preventing an excessive voltage from being applied to the gate of the switching element is required. As illustrated in FIG. 2, in a conventional asynchronous switching mode converter configured to prevent an excessive voltage from being applied, charges output from a charge pump 40 are additionally charged in a bootstrap capacitor 30 through a current source 42, and a Zener diode 44 is connected to the gate and source terminals of a switching element 24 to which the charging voltage of the bootstrap capacitor 30 is applied, thereby adjusting a voltage applied to the gate and source terminals of the switching element 24 to a uniform value. In this case, FIG. 2 illustrates the asynchronous switching mode converter, in which the low-side switch illustrated in FIG. 1 has been replaced with a diode 46.
However, the process utilizing a Zener diode is provided by only limited vendor, the prices of products utilizing a Zener diode increase. In particular, in the case of products applied to vehicles or automotives, the choice of process is more limited if one wants to use a Zener diode in an IC chip.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.